Process for activation of silver alloy catalyst



June 3, 1958 2,837,473

PROCESS FOR ACTIVATION OF SILVER ALLOY CATALYST K. E. Ma coRMAcK ETAL Filed. Oct, 10, 1956 INVENTORS KENNETH ffl/PNSI/ n MAcCfl/FMfiCK 6 321%? ANT/ION) HARROM/ W 9 I ATTORNEYS.

United States Patent PROCESS FOR ACTIVATEON F SILVER ALLOY CATALYST Application October 10, 1956, Serial No. 615,024 4 Claims. (Cl. 204-140) This invention relates to a novel method for activating silver alloy catalyst for use in the oxidation of ethylene and the like.

Objects and advantages of the invention will be set forth in part hereinafter and in part will be obvious herefrom, or may be learned by practice with the invention, the same being realized and attained by means of the processes and steps pointed out in the appended claims.

The invention herein described.

An object of our invention is to produce a novel, rapid method for activating silver alloy catalysts simply and efficiently.

Another object of our invention is to provide a novel electrolytic method for activating particular silver alloy catalysts which heretofore could not be activated by the known chemical methods.

Figures 1 and 2 of the accompanying drawings are schematic views showing different forms of apparatus for carrying out the process of the present invention.

Silver-alkaline earth alloy catalysts, as, for example, silver-calcium alloys, are known to be efiicient catalysts in the oxidation of ethylene and ethylenic hydrocarbons, as shown by the patents to Cambron et 211., 2,562,857 and 2,562,858. It is also known that removal of a substantial amount of the. alloying alkaline earth metal renders the alloy Various methods have been proposed for removing some or nearly all of the alkaline earth metal from silver- .alkaline earth metal alloy catalysts. in these methods, the reagents used react with the alkaline earth metal but do not substantially react with the silver. In one method, .the catalyst may be treated in an autoclave with water at a temperature of, say, 250 to 300 C. to convert the alkaline earth metal to its hydroxide or oxide which is re moved with the water. This procedure is slow, however, because .of the low solubility of the alkaline earth metal hydroxides and oxides in water. The removal may be facilitated by adding to the water an acid which converts the hydroxide or oxide to a more soluble salt. Care must .be taken, however, that the acid employed be inert with respect to the silver. Also, the alloy may be treated with :steam and acetic acid in accordance with the process described in the Cambron et a1. patents referred to.

Removal may also be accomplished by passing air at high temperature over the catalyst to oxidize the alkaline earth metal and subsequently treating the alloy with an acid or other agent which does not react with the silver :and which forms Water soluble salts or derivatives of the :alkaline earth metal hydroxide or oxide. Examples of :such agents that have been employed are acetic acid, formic acid, citric acid, potassium cyanide, sodium hydroxide, potassium hydrogen fluoride and other electrolytes forming soluble derivatives with the non-silver metal, in concentrations of 5 to about 50%.

Thus, heretofore, various chemical treating methods have utilized the removal of alkaline earth'metal from silver-alkaline earth metal alloys to render some catalyticonsists in the novel processes and steps highly porous and catalytically active.

activation of the ICC cally active. These methods, although useful, are rather complex and are not entirely satisfactory, requiring long periods of treatment.

Furthermore, We have found that the foregoing and other etching methods, although effective in activating silver-alkaline earth metal catalysts, are not suitable for activating silver-alloy catalysts in general as, for example, silver-aluminum alloy catalysts. Other etching techniques appropriate to some aluminum alloys, as, for example, the etching of nickel-aluminum alloys by caustic soda to produce a catalytic form of nickel, have also proved useless in the etching of silver-aluminum alloys. We have discovered that a large number of silver alloy catalysts, for use in the oxidation of ethylene, may be activated by a novel electrolytic treatment. Illustrative examples of such alloys which may be treated in accordance with our process include silver-lithium alloys, silverpotassium alloys silver-sodium alloys, silver-strontium alloys, silver-barium alloys, silver-calcium alloys, silvermagnesium alloys, silver-aluminum alloys, silver-zinc alloys, silver-cadmium alloys, and silver-tin alloys.

Our novel process in general relates to activation of silver alloys by removing the non-silver components of the alloys by selective electrolysis. Accordingly, the catalyst to be activated is deposited on a metal strip in the form of a porous layer, or is contained as small granules in a permeable bag or tube. the alloy in the form of a porous layer is by spraying the molten alloy on a metallic strip or permeable container, as, for example, a stainless steel strip, in accordance with the method described in detail in co-pending application a Serial No. 665,742, filed June 14, Alexander.

The alloy coated strip is immersed ma suitable electrolyte and a current passed therethrough, causing electrolysis, wherein the non-silver, and in this case, the more eiectro-positive component of the alloy dissolves primarily and preferentially to silver in removal or depletion of the non-silver component from the alloy by this method causes the silver to be rendered porous and catalytically active for use in the conversion of ethylene to ethylene oxide.

Generally, the electrolytic cell while the metallic strip, or cylinder surrounding the anode. This cathode may be silver, stainless steel strip, etc. Any suitable electrolyte may be used, such as acetic acid solu tion, sodium cyanide solution, sodium hydroxide solution,

1957, by Wendal A.

etc., but preferably avoiding formation of insoluble derivatives of the non-silver component.

Direct current at a low current density, preferably not 7 more than 0.3 rna./cm. is generally used in effectingihe desired electrolysis. We have found, however, that'in treating certain alloys, as for example silver-aluminum alloys, an increased removal of aluminum is attained by utilizing a simultaneous passage of direct and alternating currents at low current densities.

In carrying out our process, the current passed through the electrolyte should be of low density to permit controlled potential electrolysis, and to reduce mechanical destruction'of the porous deposit. Because of this, it is possible to' obtain selective removal of the non-silver component of the alloy coated anode, causing a high alloy. The optimum current for a particular alloy will, of course. vary depending upon the chemical composition and physical state of the alloy. Removal of any of the non-silver component creates some catalytic activity but for best results removal of from 5% or more up to removal of substantially all of the nonsilver component produces the best catalytic activity.

Figure l of the drawings illustrates diagrammatically a form of apparatus for the electrolytic activation of a One method of depositing V the electrolyte. The

the alloy coated strip forms the anode of cathode is in the form of silver alloy catalyst in accordance with the present invention. As illustrated, the silver base alloy to be catalytically activated is in the form of a sheet-metal plate 10, as the anode, surrounded by a cathode in the form of a metallic strip 12, the anode and cathode being immersed in the aqueous electrolyte 14 and connected to a suitable source of direct current 16.

Figure 2 is a similar view showing a granular catalytic alloy 18, according to the present invention, contained within a permeable tube 20 forming the anode and which, together with the cathode 12, is immersed in the electrolyte 14 and is subjected to the action of the direct current from source 16.

Alloys activated in accordance with our electrolytic method have been found to possess equivalent activity to those which can be activated by chemical methods. Moreover, our process enables us to activate silver alloy catalysts, such as silver-aluminum catalyst, which heretofore could not be activated by the known chemical etching methods.

A large number of specific examples are disclosed below in Tables i to Ill; Table I relating to silver-calcium alloys; Table ll to silver-zinc alloy catalysts; and Table III to silver-aluminum catalysts.

TABLE I Silver calcium alloy containing initially 8.5% Ca Anode-Stainless steel strip with coating of silver calcium alloy Cathode-Silver gauze Initial Percent Final weight Current theoreticomposiper- Electrolyte Temp, density, cal tion, cent C. maJeru. current weight- Oa passed percent 8.5-... 20% acetic acid 28 1.3 .3 1.91 8.5 do 28 .3 97 .12 8.5 d 28 1.4 33 1.0 8.5- acetic acid and 28 1. 4 33 2.1

Versene F83. 8.5- 20% acetic acid 28 1.4 33 1. 31 8.5. 20% acetic acid and 28 1. 4 33 1. 23

excess oxalic acid. 8.5- 20% acetic acid 70-90 1. 4 33 1. 08

TABLE I1 Silver zinc alloy containing llllltfllly 21.4% Zinc Anode-Stainless steel strip with coating of silver zinc alloy CathodeSilver gauze Initial Percent Final weight Current theoreticompoper- Electrolyte Temp, density, cal sit-ion, cent C. min/em. current weight Zn passed percent 21.4. 20% acetic acid 28 1. 4 72 19. 0 21.4..- d0. .28 1.4 72 13.7 21.4- 2 N KC 28 1.4 72 10. 4 21 20% aceti 70-90 1. 4 72 15. 9 21.4- 2 N NaOH. 70-90 1.4 72 16.4

4 TABLE III Silver aluminum alloy containing 11.15% Al AnodeStainless steel strip coated with silver-aluminum alloy CathodcStoiuless steel strip NOTE.-Ln the last two examples 4 volt, cycle A. C. was super imposed on the D. 0. used.

The invention in its broader aspects is not limited to the specific processes and steps described, but departures may be made therefrom within the scope of the accompanying claims Without departing from the principles of the invention and without sacrificing its chief advantages.

We claim:

1. A process for the activation of silver alloy catalyst, comprising a porous mass of alloy of silver and a nonsilver component selected from the group consisting of silver-lithium, silver-potassium, silver-sodium, silver-strontium, silver-barium, silver-calcium, silver-magnesium, silver-aluminum, silver-zinc, silver-cadmium and silvertin alloys, comprising immersing the alloy in an aqueous electrolyte and passing current therethrough with the alloy as an anode, thereby causing selective removal of at least some of the non-silver components to form an activated catalyst.

2. A process as defined in claim 1, wherein the silver alloy is in the form of a porous layer deposited on a metallic strip, said strip forming an anode when the current is passed through the electrolyte solution.

3. A process as claimed in claim 1 in which the porous mass of alloy is in the form of fine particles supported in a permeable container surrounding the anode.

4. A process as defined in claim 1, wherein the electrolyte is selected from the group consisting of acetic acid, sodium hydroxide and potassium cyanide solution.

References Cited in the file of this patent UNITED STATES PATENTS 490,816 Theuerner Ian. 31, l893 1,590,897 Lush June 29, 1.926 2,253,835 Wellman Aug. 26, l94l 2,253,871 Semon Aug. 26, I941 

1. A PROCESS FOR THE ACTIVATION OF SILVER ALLOY CATALYST, COMPRISING A POROUS MASS OF ALLOY OF SILVER AND A NONSILVER COMPONENT SELECTED FROM THE GROUP CONSISTING OF SILVER-LITHIUM, SILVER-POTASSIUM, SILVER-SODIUM, SILVER-STRONTIUM, SILVER-BARIUM, SILVER-CALCIUM, SILVER-MAGNESIUM, SILVER-ALUMINUM, SILVER-ZINC, SILVER-CADMIUM AND SILVERTIN ALLOYS, COMPRISING IMMERSING THE ALLOY IN AN AQUEOUS ELECTROLYTE AND PASSING CURRENT THERETHROUGH WITH THE ALLOY AS AN ANODE, THEREBY CAUSING SELECTIVE REMOVAL OF AT LEAST SOME OF THE NON-SILVER COMPONENTS TO FORM AN ACTIVATED CATALYST. 